The six biggest mysteries of our solar system

ONCE upon a time, 4.6 billion years ago, something was brewing in an unremarkable backwater of the Milky Way. The ragbag of stuff that suffuses the inconsequential, in-between bits of all galaxies - hydrogen and helium gas with just a sprinkling of solid dust - had begun to condense and form molecules. Unable to resist its own weight, part of this newly formed molecular cloud collapsed in on itself. In the ensuing heat and confusion, a star was born - our sun.

We don't know exactly what kick-started this process. Perhaps, with pleasing symmetry, it was the shock wave from the explosive death throes of a nearby star. It was not, at any rate, a particularly unusual event. It had happened countless times since the Milky Way itself came into existence about 13 billion years ago, and in our telescopes we can see it still going on in distant parts ...

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